Seven factors that affect the screening effect
When the particle size is close to the mesh size, the screening efficiency is significantly reduced. The overall sieving efficiency decreases significantly with the increase in the proportion of particles near the sieve opening. The effect of near-mesh particles is manifold, as these particles restrict or block the mesh openings, reducing the effective screening area of the sieve. This is a process that often occurs in the sieve in the closed-circuit crushing process. In the closed-circuit process, the materials near the sieve hole gradually increase, which gradually reduces the screening efficiency.
2. Feeding speed
The analytical principle of sieving resolution is that almost complete separation can be achieved using lower feed rates and longer sieving times. In the screening practice, considering the economic cost, it is required to adopt a higher feeding speed and reduce the residence time of the material on the screen surface. At higher feeding speeds, a thick layer of material is formed on the screen surface, and fine particles must pass through the thick layer to have a chance to pass further through the screen surface. The net result is a reduction in screening efficiency. For any separation operation, high output and high efficiency are often two contradictory requirements. To obtain the best results, the above two indicators must be comprehensively considered.
3. Sieve inclination
If the particles are close to the sieve opening at a small inclination angle, the sieve “seems” to be an effective pore size, and the particles near the sieve opening can hardly pass through. The inclination of the sieve surface affects the angle at which the particles enter the sieve hole, and some sieves use this effect to separate finer materials than the sieve hole. The sieve angle also affects the speed at which the particles move along the sieve surface, the residence time on the sieve surface, and the probability of the particle passing through the sieve surface.
4. Particle shape
Most of the material particles processed on the sieve are non-spherical. When the spherical particles pass through with equal probability in all directions, the irregular-shaped particles near the sieve hole must pass through the sieve hole in a certain direction. Elongated and tabular particles have smaller cross-sectional areas in one direction and larger cross-sectional areas in other directions. Consequently, the sieving efficiency of particles of very irregular shapes is reduced.
5. Effective screening area
The probability of passing the sieve is proportional to the percentage of the effective sieving area of the sieve. The effective sieving area refers to the ratio of the sieve hole area to the total area of the sieve surface. The smaller the screen surface occupied by the screen material, the greater the probability of particles entering the screen holes. The effective sieving area decreases as the sieve size decreases. Therefore, to increase the effective area of the small-hole sieve, a fine and brittle sieve wire must be used, but its sieve is easy to wear and the processing capacity is low, which is why the fine-grained material is classified by a classifier instead of a fine sieve.
To throw the material off the screen surface, the screen should be vibrated, and the vibration causes the material to fall back to the screen surface again and move along the screen surface. Appropriate vibration can also cause the stratification of the feed, which causes the fine particles to pass through the material layer and down to the screen surface, thereby causing the large particles to rise to the top. The layering action increases the pass rate in the middle part of the sieve.
The surface moisture content of the feed has a significant effect on the screening efficiency, just as it does with clay and other cohesive materials. Wet materials are difficult to screen, so do not screen wet materials to prevent clogging of the screen holes. These measures also include the use of thermal blankets to break the surface tension of the water between the screen and the material, the use of spherical plates to generate nearby vibrations in the lower part of the screen, or the use of non-clogging screen weave. Wet sieves can effectively screen 250um or even finer-grained materials. The sticky sludge can be washed off the large particles, so the slurry flow and flushing water can rinse the screen clean.
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